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Enhanced Outcrossing, Directional Selection and Transgressive Segregation Drive Evolution of Novel Phenotypes in Hybrid Swarms of the Dutch Elm Disease Pathogen Ophiostoma novo-ulmi

Forest Research, Alice Holt Lodge, Farnham, Surrey GU10 4LH, UK
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Academic Editor: Katrina Maria Ramonell
J. Fungi 2021, 7(6), 452; https://doi.org/10.3390/jof7060452
Received: 4 May 2021 / Revised: 2 June 2021 / Accepted: 3 June 2021 / Published: 6 June 2021
(This article belongs to the Section Fungal Evolution, Biodiversity and Systematics)
In the 1970s, clones of the two subspecies of Ophiostoma novo-ulmi, subsp. americana (SSAM) and subsp. novo-ulmi (SSNU) began to overlap in Europe, resulting in hybrid swarms. By 1983–1986, hybrids with high, SSAM-like growth and pathogenic fitness comprised ~75% of popula-tions at Limburg, Netherlands and Orvieto, Italy. We resampled these populations in 2008 to examine trends in hybrid fitness traits. Since preliminary sampling in 1979–1980, MAT-1 locus frequency had increased from ~0% to ~32% at Orvieto and 5% to ~43% at Limburg, and vegeta-tive incompatibility type frequency had changed from near clonal to extremely diverse at both sites. This represents an enormous increase in outcrossing and recombination potential, due in part to selective acquisition (under virus pressure) of MAT-1 and vic loci from the resident O. ulmi and in part to SSAM × SSNU hybridisation. Overt virus infection in the 2008 samples was low (~4%), diagnostic SSAM and SSNU cu and col1 loci were recombinant, and no isolates exhib-ited a parental SSAM or SSNU colony pattern. At both sites, mean growth rate and mean patho-genicity to 3–5 m clonal elm were high SSAM-like, indicating sustained directional selection for these characters, though at Orvieto growth rate was slower. The once frequent SSNU-specific up-mut colony dimorphism was largely eliminated at both sites. Perithecia formed by Limburg isolates were mainly an extreme, long-necked SSNU-like form, consistent with transgressive segregation resulting from mismatch of SSAM and SSNU developmental loci. Orvieto isolates produced more parental-like perithecia, suggesting the extreme phenotypes may have been se-lected against. The novel phenotypes in the swarms are remodelling O. novo-ulmi in Europe. Locally adapted genotypes may emerge. View Full-Text
Keywords: hybridisation; pandemic; invasive species; fitness traits; perithecia; mating type; vegetative compatibility; pathogenicity hybridisation; pandemic; invasive species; fitness traits; perithecia; mating type; vegetative compatibility; pathogenicity
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MDPI and ACS Style

Brasier, C.; Franceschini, S.; Forster, J.; Kirk, S. Enhanced Outcrossing, Directional Selection and Transgressive Segregation Drive Evolution of Novel Phenotypes in Hybrid Swarms of the Dutch Elm Disease Pathogen Ophiostoma novo-ulmi. J. Fungi 2021, 7, 452. https://doi.org/10.3390/jof7060452

AMA Style

Brasier C, Franceschini S, Forster J, Kirk S. Enhanced Outcrossing, Directional Selection and Transgressive Segregation Drive Evolution of Novel Phenotypes in Hybrid Swarms of the Dutch Elm Disease Pathogen Ophiostoma novo-ulmi. Journal of Fungi. 2021; 7(6):452. https://doi.org/10.3390/jof7060452

Chicago/Turabian Style

Brasier, Clive, Selma Franceschini, Jack Forster, and Susan Kirk. 2021. "Enhanced Outcrossing, Directional Selection and Transgressive Segregation Drive Evolution of Novel Phenotypes in Hybrid Swarms of the Dutch Elm Disease Pathogen Ophiostoma novo-ulmi" Journal of Fungi 7, no. 6: 452. https://doi.org/10.3390/jof7060452

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